Fix a data_workers test
Fix a data_workers test

Summary:
This is a global variable which can be incremented by other tests.

Before:
```
$ pytest -v caffe2/python/data_workers_test.py
...
caffe2/python/data_workers_test.py::DataWorkersTest::testGracefulShutdown PASSED
caffe2/python/data_workers_test.py::DataWorkersTest::testNonParallelModel FAILED

============================================= FAILURES ==============================================
_______________________________ DataWorkersTest.testNonParallelModel ________________________________

self = <data_workers_test.DataWorkersTest testMethod=testNonParallelModel>

    def testNonParallelModel(self):
        model = cnn.CNNModelHelper(name="test")
        coordinator = data_workers.init_data_input_workers(
            model,
            ["data", "label"],
            dummy_fetcher,
            32,
            2,
        )
>       self.assertEqual(coordinator._fetcher_id_seq, 2)
E       AssertionError: 4 != 2

caffe2/python/data_workers_test.py:38: AssertionError
-----------------
Closes https://github.com/caffe2/caffe2/pull/211

Differential Revision: D4916591

Pulled By: Yangqing

fbshipit-source-id: 281f12d7f02dbd0ce0932024cf1f16cd12130112

pietern/caffe2,bwasti/caffe2,xzturn/caffe2,Yangqing/caffe2,pietern/caffe2,pietern/caffe2,davinwang/caffe2,Yangqing/caffe2,davinwang/caffe2,bwasti/caffe2,Yangqing/caffe2,bwasti/caffe2,sf-wind/caffe2,bwasti/caffe2,davinwang/caffe2,xzturn/caffe2,sf-wind/caffe2,xzturn/caffe2,pietern/caffe2,Yangqing/caffe2,caffe2/caffe2,Yangqing/caffe2,xzturn/caffe2,pietern/caffe2,davinwang/caffe2,sf-wind/caffe2,sf-wind/caffe2,davinwang/caffe2,xzturn/caffe2,sf-wind/caffe2,bwasti/caffe2